The present invention relates to semiconductor devices and fabrication methods thereof, and particularly relates to a semiconductor device including a trapping film for charge accumulation and word lines which are provided in a MONOS nonvolatile semiconductor memory device and a fabrication method thereof.
MONOS (metal-oxide-nitride-oxide-semiconductor) nonvolatile semiconductor memory devices are nonvolatile semiconductor memory devices for accumulating charges in an ONO film of a layered structure in which a silicon oxide layer, a silicon nitride layer, and a silicon oxide layer are layered sequentially. Of various types of MONOS nonvolatile semiconductor memory devices proposed heretofore, a nonvolatile semiconductor memory element including an ONO film for storing information by locally accumulating charges receives attention for its suitability to increase density and performance and to reduce power consumption. This nonvolatile semiconductor element includes bit lines formed in a semiconductor substrate, the ONO film formed on a channel region, and word lines formed on the bit lines so as to be intersected at a right angle with the bit lines.
Such a conventional nonvolatile semiconductor memory device will be described with reference to FIG. 6A to FIG. 6D, and a fabrication method thereof will be described with reference to FIG. 7A(a) to FIG. 7A(a), FIG. 7B(a) to FIG. 7B(e), and FIG. 7C (see, for example, Japanese Patent Application Laid Open Publication No. 2001-77220A and Japanese Patent Application Laid Open Publication No. 2000-91450A).
FIG. 6A is a plan view of the conventional semiconductor device which includes bit lines 607 and word liens (gate electrodes) 603 formed on the bit lines 607 so as to be intersected at a right angle with the bit liens 607.
As shown in FIG. 6B, which is a sectional view taken along the line A-A′ in FIG. 6A, there are provided: an ONO film 602 as a trapping film on a first conductivity type semiconductor substrate 601; word lines 603 on the ONO film 602; first silicon nitride films 604 formed by low pressure CVD on the side faces of the word lines 603 and the side face of the ONO film 602; and a second silicon nitride film 605 formed by plasma CVD or low pressure CVD and covering the surfaces of the word lines 603, the surfaces of the first silicon nitride films 604, and the surface of the semiconductor substrate 601.
As shown in FIG. 6C, which is a sectional view taken along the line B-B′ in FIG. 6A, there are provided: the ONO film 602 as a trapping film on the first conductivity type substrate 601; a plurality of opening portions formed in the ONO film 602 in the lateral direction of the word lines 603; second conductivity type diffusion bit lines 607 in regions of the semiconductor substrate 601 which are located below the openings; oxide insulating films 606 on the diffusion bit lines 607; the word lines 603 on the surface of the ONO film 602, the side face of the ONO film 602, and the surfaces of the oxide insulating films 606; and the second silicon nitride film 605 formed by plasma CVD or low pressure CVD and covering the surfaces of the word lines 603.
As shown in FIG. 6D, which is a sectional view taken along the line C-C′ in FIG. 6A, there are provided: the second conductivity type diffusion bit lines 607 in the first conductivity type semiconductor substrate 601; the oxide insulating films 606 on the diffusion bit lines 607; the word lines 603 on the oxide insulating films 606; the first silicon nitride films 604 formed by low pressure CVD on the side faces of the word lines 603 and the side faces of the oxide insulating films 606; and the second silicon nitride film 605 formed by plasma CVD or low pressure CVD and covering the surfaces of the word lines 603, the surfaces of the first silicon nitride films 604, and the surfaces of the diffusion bit lines 607.
A conventional semiconductor device fabrication method will be described next with reference to FIG. 7A, FIG. 7B, and FIG. 7C which correspond to the sectional views taken along the lines A-A′ and B-B′ in FIG. 6.
First, as shown in FIG. 7A(a), an ONO film 702 as a trapping film is formed on a first conductivity type semiconductor substrate 701.
Next, as shown in FIG. 7A(b), a resist pattern 703 that defines the positions to be a plurality of bit lines are formed on the ONO film 702.
Subsequently, as shown in FIG. 7A(c), the upper part of the ONO film 702 is removed with the use of the resist pattern 703 as a mask. Though the lower part of the ONO film 702 is left herein, the ONO film 702 is removed until the semiconductor substrate 701 is exposed in some cases.
Thereafter, as shown in FIG. 7A(d), second conductivity type impurity ion is implanted to the semiconductor substrate 701 with the use of the resist pattern 7103 as a mask though the remaining lower part of the NON film 702 to form a plurality of second conductivity type diffusion layers 704 in regions of the semiconductor substrate 701 which are located below the openings of the resist pattern 703.
Next, the resist pattern 703 is removed, as shown in FIG. 7A(e), and then, accelerated oxidation is performed on each diffusion layer 704, as shown in FIG. 7A(f). The accelerated oxidation forms an oxide insulating film 705 formed of a silicon oxide film in the upper part of each diffusion layer 704 while at the same time activating the implanted impurity ion to thus form bit lines 706 formed of the diffusion layers 704.
Subsequently, as shown in FIG. 7A(g), conductive polysilicon 707 is deposited on the surface of the ONO film 702, the side face of the ONO film 702, and the surfaces of the oxide insulating films 705.
Thereafter, as shown in FIG. 7B(a), a resist pattern 708 that defines positions to be a plurality of word lines so that the word lines are intersected at a right angle with the bit lines 706 is formed on the surface of the thus deposited polysilicon 707.
Next, as shown in FIG. 7B(b), the polysilicon 707 is removed with the use of the resist pattern 708 as a mask to form word lines (gate electrodes) 709 formed of the polysilicon 707. Herein, the ONO film 702 is removed until the semiconductor substrate 701 is exposed.
Subsequently, the resist pattern 708 is removed, as shown in FIG. 7B(c), and then, a first silicon nitride film 710 is formed by low pressure CVD on the surfaces of the word lines 709, the side faces of the word lines 709, the side face of the ONO film 702, and the surface of the semiconductor substrate 701, as shown in FIG. 7B(d).
Thereafter, as shown in FIG. 7B(e), the silicon nitride film 710 is removed by anisotropic etching to form sidewalls 711 formed of the first silicon nitride film 710 on the side faces of the word lines 709 and the side face of the ONO film 702.
Finally, as shown in FIG. 7C, a second silicon nitride film 712 is formed by plasma CVD or low pressure CVD so as to cover the surfaces of the word lines 709, the surfaces of the sidewalls 711, and the surface of the semiconductor substrate 701. Thus, the conventional nonvolatile semiconductor memory device is completed.